1. Field of the Invention
The present invention relates to a mobile communication system, and more particularly to the control of forward link power in a CDMA mobile communication system.
2. Description of the Prior Art
In a mobile communication system using a code division multiple access (CDMA), such as a cellular mobile communication system or a personal communication service system, a plurality of mobile stations transmits and receives frames via a base station or a cell-site. The communication path used by a base station to transmit data frames to a user is called a forward link. Conversely, the reverse link refers to the communication path used by a user to transmit data frames back to a base station.
Each of the frames is composed of symbols indicating a digitized voice and related data. These frames may fade during transmission through a multipath because reflections in the transmission environment as the mobile stations move. The frames may also fade by an interference of one specific mobile station with another mobile stations due to a high transmission power.
Also, if a certain mobile station is extremely low in power, the associated base station cannot recover the data of a frame transmitted from the mobile station, resulting in a loss of data. Thus, the base station has to control the frame transmission power of the mobile station to adequately recover the data of the transmitted frame while minimizing the fading of the transmitted frames.
For frame transmission power control, the base station is adapted to measure the transmission power of the frame transmitted from the mobile station and send a power control command to the mobile station according to the measured result. The power control command instructs the mobile station to adjust the transmission power through separate channels.
This power control command is composed of a transmission power increment bit or a transmission power decrement bit for maintenance of an average reception power.
FIG. 1 is a block diagram showing a portion of a conventional apparatus for controlling a reverse link power in a CDMA mobile communication system. A spread-band CDMA signal transmitted from a mobile station (not shown) is received at a receiving antenna 20 of a base station, generally through a reverse link CDMA channel. A radio frequency (RF) receiver 21 performs a frequency down process with respect to the received CDMA signal.
An analog/digital (A/D) converter 22 converts the CDMA signal frequency down-processed by the RF receiver 21 into a digital signal, and a PN correlator 23 performs a correlation process with respect to the digitized CDMA signal from the A/D converter 22 on the basis of a PN code provided from a PN code generator (not shown). The PN correlator 23 supplies an output signal to a decoder 24 which performs a decoding operation and to a power estimator 25 which estimates the average reception power of the mobile station transmitting the CDMA signal.
The decoder 24 decodes the output signal from the PN correlator 23 into a code symbol of the received CDMA signal and also supplies code error metrics indicating the quality of the received CDMA signal to an outer loop power controller 26. In response to the code error metrics from the decoder 24, the outer loop power controller 26 detects a sufficient power of the mobile station to transmit data and supplies a power control reference value for the control of reverse link power to a comparator 27.
The comparator 27 compares the average reception power of the mobile station estimated by the power estimator 25 with the power control reference value from the outer loop power controller 26 and supplies a deviation of the average reception power from the power control reference value to a power control bit generator 28. The power control bit generator 28 generates a reception power increment bit or a reception power decrement bit according to the deviation supplied from the comparator 27 and provides the generated reception power increment bit or reception power decrement bit to a transmitter 29.
The transmitter 29 transmits the reception power increment bit or reception power decrement bit from the power control bit generator 28 together with the frequency up-processed user data to the mobile station through a transmitting antenna 30 while placing them on a traffic channel among the CDMA channels of a forward link.
The CDMA channels in the forward link are generally a pilot channel, a synchronous channel, a paging channel and a traffic channel for forward connection and communication from the base station to the mobile station. The CDMA signals through these CDMA channels are orthogonally processed based upon Walsh function codes and processed as orthogonal pairs of a pseudo noise (PN) sequence based upon a fixed chip rate such that the signals can be code-symbolized.
Generally, a CDMA signal may be transmitted at various rates of 9.6 kbps (full rate), 4.8 kbps (half rate), 2.4 kbps (quarter rate) and 1.2 kbps (eighth rate). The CDMA signal is also encoded in the mobile station. Typically, the CDMA signal is encoded into a frame at an interval of 20 msec. An error correcting code is used in the encoding process to automatically correct an error of the digital data.
Referring back to FIG. 1, the power control through the power estimator 25, the comparator 27 and the power control bit generator 28 forms a closed loop power control. In the closed loop power control, the base station measures a transmission rate-based reception power of a signal received from the mobile station, compares the measured reception power with a predetermined power control reference value and sends an appropriate power control command as a result of the measurement together with user data to the mobile station. Thus, the mobile station adjusts the transmission power in response to the sent power control command and transmit a signal at the adjusted transmission power.
In the closed loop power control, the power control reference value is determined differently for each mobile stations depending on various parameters such as the moving speeds of the mobile station, the surroundings of the mobile stations, etc. Particularly, the outer loop power controller 26 in the base station adjusts the power control reference value appropriately according to a frame error rate of the received data. Such power control is called an outer loop power control.
As discussed above, the conventional power control of the CDMA mobile communication system is mainly a reverse link control. Particularly, the mobile station aids the base station in the control of the power on the forward channel by transmitting a power control message to the base station on the reverse link. The mobile station estimates the reception power and informs the base station via the power control message.
The base station may accordingly adjust the transmission power, i.e. the reception power at the mobile station, to each mobile station. Through the power control message, the base station may minimize the necessary transmission power to each individual mobile station for maintaining a link of a specified quality. By minimizing each transmission power, the total interference generated is also minimized, thereby improving the capacity of the system.
In the conventional CDMA mobile communication system, the mobile station evaluates its error performance and sends a power control message for each frame. Thus, the base station adjusts the transmission power output every frame or 50 Hz. The power output to each mobile station is updated on a frame by frame basis through a feedback mechanism from the mobile station to the base station. The feedback mechanism allows the mobile station to inform the base station on whether the frames are received correctly or incorrectly by including such information in every frame of data transmitted to the base station.
However, the conventional power control for the CDMA mobile communication system fundamentally limits the rate (50 Hz) in which a mobile station sends a power control message and the rate in which the base station adjusts the power out to the mobile station. A high speed transmission power adjustment would allow the base station to efficiently minimize the necessary transmission power to each individual mobile station for maintaining a link of a specified quality, thereby efficiently minimizing the total interference generated.
Thus, a high speed power control is required for a more efficient CDMA communication system. A forward link power control would allow a higher speed power control. For a high-speed forward link power control, a mobile station must measure the power of a CDMA signal received from a base station and determine which one of the CDMA channels is to be observed and used for the power control.
Assuming that a forward link power control is performed by observing the traffic channel similarly as the reverse link power control, the following problem occurs. Namely, the traffic channel of the forward link is variable in transmission power according to a transmission rate. As the data transmission rate changes, the base station changes the transmission power by an amount equivalent to the change in the data transmission rate. Since the new frame rate is not known to the mobile station, the mobile station cannot distinguish whether the reception power changes are due to fading or rate changes.
For this reason, when the power control is performed by judging a transmission rate of a signal currently received from the mobile station, and calculating a signal-to-interference ratio according to the judged result, a considerable amount of transmission delay occurs. Thus the base station cannot follow the currently varying CDMA channel environments.
One method in the related art for estimating the reception power on the forward link utilizes punctured power control symbols. In this method, the power control message bits that are sent on the forward link to control the transmission power of the mobile station in the reverse link are punctured on the forward traffic channel at the rate of 800 Hz. The bits for power control message is fixed. As a result, the power control can be performed by observing the power control bit transmitted through the traffic channel. However, because the power control bit is very short in length, a significant error may arise in calculating an interference ratio to the currently received CDMA signal, resulting in an increase in the probability of an erroneous power control.
Moreover, a power control performed by observing the pilot channel rather than the traffic channel cannot be performed independently by the plurality of mobile stations belonging to the base station, because the pilot channel is commonly used by the mobile stations.
Accordingly, an object of the present invention is to solve at least the problems and disadvantages of the related art.
An object of the present invention is to provide a fast forward link power control in a CDMA mobile communication system.
Additional advantages, objects, and features of the invention will be set forth in part in the description which follows and in part will become apparent to those having ordinary skill in the art upon examination of the following or may be learned from practice of the invention. The objects and advantages of the invention may be realized and attained as particularly pointed out in the appended claims.
To achieve the objects and in accordance with the purposes of the invention, as embodied and broadly described herein, an apparatus for controlling forward link power in a CDMA mobile communication system comprises a traffic channel correlator performing a first correlation operation with respect to a code symbol of a CDMA signal received through a traffic channel; a pilot channel correlator for a second correlation operation with respect to a code symbol of the CDMA signal received through a pilot channel; a power control bit extractor extracting a power control bit from the code symbol received through the traffic channel, if the transmission rate remains consistent to a correlation value from the traffic channel correlator; a power estimator estimating the average of reception power in response to the power control bit extracted by the power control bit extractor and the correlation value from the pilot channel correlator; a comparator comparing the average reception power estimated by the power estimator with a predetermined power control reference value and generating a deviation of the average reception power from the predetermined power control reference value; and a power control bit generator generating the final power control bit according to the deviation generated by the comparator.
In another embodiment of the present invention, a method of controlling forward link power in a CDMA mobile communication system comprises the step of performing a forward link power control operation by using both a traffic channel and pilot channel of a forward link.
Preferably, the forward link power control operation is performed by using power control bits of code symbols transmitted through the traffic channel and the pilot channel for a predetermined time. Also, the forward link power control operation is performed by taking into consideration that a ratio of the traffic channel power to the pilot channel power obtained for a first predetermined time is equivalent to a ratio of the traffic channel power to the pilot channel power obtained for a second predetermined time.
In another embodiment of the present invention, the forward link power control operation may be performed by taking into consideration that a difference between the traffic channel power per a first predetermined time and the traffic channel power per a second predetermined time is equivalent to a difference between the pilot channel power per the first predetermined time and the pilot channel power per the second predetermined time.
In the present invention, a power control group observes code symbols transmitted through the traffic channel and pilot channel for a first predetermined time. The power control group is adapted to observe the code symbols for a second predetermined time where the probability that the code symbols are present is high. Moreover, the power control group is divided to perform a power estimation operation regarding the observed code symbols and other processes for a third predetermined time being equal to a difference of the second predetermined time from the first predetermined time.
In the present invention, a current power control group observes code symbols transmitted through the traffic channel and pilot channel for a first predetermined time. The current power control group is adapted to observe the code symbols for a second predetermined time where the probability that the code symbols are present is high and for a third predetermined time remaining after observation in a previous power control group. Moreover, the current power control group is divided to perform a power estimation operation regarding the observed code symbols and other processes for a fourth predetermined time remaining after observation in the current power control group. The third predetermined time is equal to a difference of the second predetermined time from the first predetermined time, and the fourth predetermined time is equal to the third predetermined time.